1. Field of the Invention
The present invention relates to a steering device with a constitution in which a operation unit operated by a driver and steerable wheels which are steered are not mechanically connected, and may be made to connect from such a condition.
Priority is claimed on Japanese Patent Application No. 2004-252087, filed Aug. 31, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
Conventionally, in a so-called SBW (steer-by-wire) type steering device, there is no mechanical connection between the steering wheel (hereinafter also referred to as an operation unit) steered by the driver, and the steerable wheels (the vehicle wheels). Therefore, greater output is required of the turning actuator which actually applies the turning angle to the steerable wheels, than in a conventional steering system in which the steering wheel and the steerable wheels are linked mechanically, due to the fact that steering effort of the driver applied to the steering wheel is not directly transmitted to the turning actuator.
Furthermore, in an SBW type steering device, the physical rack end position on the steerable wheels side cannot be felt at the steering wheel side by the driver. Therefore, it is necessary to otherwise make the driver aware of so-called rack end bottoming that corresponds to the maximum turning angle.
One known method of making the driver aware of the maximum turning angle is to rapidly build up the steering reaction force when the steering angle (steering wheel angle) approaches an angle at which the turning angle reaches the maximum, thereby letting the driver know when the turning angle reaches the maximum (see, for example, U.S. Pat. No. 5, 247,441).
Incidentally, in the case of an SBW type steering device in which an electric motor is used for the turning actuator, there is a need for the output capacity of the turning actuator to be as low as possible from the viewpoint of decreasing manufacturing costs and power consumption.
However, if the output of the motor is not sufficient, the driver may be unable to adequately turn the steerable wheels to the turning angle intended by the driver, i.e., the target turning angle determined according to the steering angle, so that the driver experiences considerable unusual sensation. This unusual sensation is particularly marked at extremely low speeds where high turning output is required, such as when parking, for example.
Moreover, when the steerable wheels do not sufficiently approach the target turning angle according to the steering operation by the driver, i.e., when a motor of the turning actuator turns the steerable wheels at the maximum output, a phenomenon can occur in which, even if the driver reverses the steering direction of the steering wheel, the steerable wheels continue to be turned in the turning direction in which the steerable wheels had been turned in the first place.
At this time, even if the driver does not have a feeling that the steerable wheels are not responding properly, when the state is continued in which the steering direction of the steering wheel and the turning direction of the steerable wheels are reversed with respect to each other, a vehicular motion is showed in which the vehicle continues to be turned in the same direction (direction opposite to the steering direction) as the direction in which the vehicle has been turned until then, and after that, the vehicle begins to be turned in the same direction (same direction as the steering direction) as the direction of the intention-of the driver, and the driver will feel considerable unusual sensation.